In forming a semiconductor substrate, compared with the technique of building the integrated circuit into a bulk semiconductor substrate, the technique of forming various devices on a thin film semiconductor layer provided on an oxide insulating layer gives superior operating speeds, .alpha.-ray blocking properties, and other device properties and further is more advantageous in terms of the isolation of the devices. This type of semiconductor substrate is called an SOI (silicon on insulator) structure substrate. In such an SOI structure substrate, the thinner the active layer forming the devices is made, the more the parasitic capacitance of the PN junction can be reduced and the higher the operating speed of the devices can be made, so studies have been made on making superthin active layers.
As one technique for-obtaining a thin film SOI structure substrate, the so-called binding method is known. FIGS. 7a to 7e are cross-sectional views showing the process for production of an SOI structure substrate by the conventional binding method.
In this process of production, first, an oxide film 3 comprised of SiO.sub.2 is formed on the surface of a silicon substrate 1 (hereinafter also referred to as the active layer substrate 1 or active layer substrate A) which has been patterned (shown as the pattern 2) (see FIG. 7a). A bonding layer 4 comprised of polycrystalline silicon is then formed on this oxide film 3 (see FIG. 7b).
Next, the surface of the bonding layer 4 is subjected to chemical-mechanical polishing to flatten it (see FIG. 7c), another silicon substrate (hereinafter also referred to as the support substrate 5 or the support substrate B) is laminated on the polishing face (bonding surface) 4a (see FIG. 7d), then the surface of the active layer substrate 1 is ground and selectively polished until the oxide film 3 is exposed (see FIG. 7e). In this way, it is possible to obtain an SOI structure substrate.
The polycrystalline silicon layer 4 of the SOI structure substrate produced by the conventional process is a layer formed with the objective of burying and flattening the step differences in the patterned oxide film and does not perform any special function after the support substrate is laminated.
In the conventional binding process, it was not possible to eliminate the step of forming the polycrystalline silicon layer (FIG. 7b) so long as the oxide film had step differences in that it was necessary to obtain a flat bonding surface 4a.
Therefore, the inventor of this application took note of the fact that if a satisfactory degree of flatness could be obtained by directly polishing the silicon oxide film formed on the surface of the silicon substrate, it would be possible to eliminate the step of forming the polycrystalline silicon layer. He engaged in intensive research and studies on techniques for polishing a silicon oxide film and as a result confirmed that by suitably constituting the polishing agent and process of polishing, it was possible to obtain a polished face able to satisfy the requirement for flatness in binding and thereby perfected the first aspect of this invention.
Further, he confirmed that by suitably constituting the polishing step using a polishing agent with a different ratio of selective polishing between the silicon and oxide film, it was possible to obtain a polished face able to satisfy the requirement for flatness in binding and thereby perfected the second aspect of this invention.